Method for removing impurities in the bayer process

ABSTRACT

Improvement in the Bayer process wherein a first cycle, alumina hydrate is precipitated from an alumina pregnant caustic liquor which liquor also contains an oxalate impurity. The precipitation of the alumina hydrate is favored and the precipitation of the oxalate in the liquor is inhibited by selectively controlling the temperature of the liquor as a function of the concentration of the oxalate contaminating impurity in the liquor.

Unite States atent Sato et al. 1 Mar. 14, 1972 [54] METHOD FOR REMOVING[56] References Cited IMPURITIES IN THE BAYER PROCESS UNITED STATESPATENTS [721 gi z l gfi fgi g 2,935,376 5/1960 Roberts ..23/143 p3,372,985 3/1968 Roberts et al ..23/l43 [73] Assignee: Showa DenkoKabushiki Kaisha, Tokyo,

Japan Primary ExaminerM. Weissman [22] Filed: Aug. 21, 1968Attorney-George B. Oujevolk [21] Appl. No.: 754,176 [57] ABSTRACT [30]Foreign Application Priority Data Improvement in the Bayer processwherein a first cycle, alumina hydrate is precipitated from an aluminapregnant caustic Aug. 23, 1967 Japan ..42/53888 liquor which liquor alsoContains an Oxalate impurity The 2 precipitation of the alumina hydrateis favored and the precipitation of the oxalate in the liquor isinhibited by sele 58] i 23,143 52 tively controlling the temperature ofthe liquor as a function of 2 V a a 3 a I Q E ELQW.

CLASSIFIER A-PROCESS 4 new SEED 5 SPENT LIQUOR H 6 DILUTE sownow,

the concentration of the oxalate contaminating impurity in the liquor.

4 Claims, 4 Drawing Figures C-PROCESS E-PROCESS THICKENER 22 a H ,YL IELPAIENTEDMAR 14 I972 SHEET 3 OF 3 225 -3 535 g: Ewan INVENTORS 5mm EEG:

@ TN w ATTORNEY METHOD FOR REMOVING IMPURITIES IN THE BAYER PROCESS Thisinvention relates to a method for removing impurities composed mainly ofsodium oxalate or oxalate salt (organic materials in the liquor asdetermined by a potentiometric titration with perchlorato-cerate inperchloric acid solution) in Bayer process and further to recovercaustic soda or sodium oxalate by giving suitable treatment to theremoved impurities.

In the Bayer process, the bauxite slurry which contains variousimpurities existing in the raw material, especially organic substancescontained in the ore and such organic substances as organic substancesedimentation promoters used in the red mud sedimentating process, istreated at a high temperature and high alkalinity in order to extractalumina.

In the process of treatment, part of the organic substances contained inthe slurry as impurities are dissolved and converted into substances oflower molecular weights such as oxalic, acetic, glycolic, carbonic orother sodium salts.

Among the above sodium salts, sodium oxalate crystallizes in the aluminamanufacturing process and deposits as scale on the inner walls ofvessels and piping, disturbing smooth operation and causing varioustroubles. Besides, if crystals of sodium oxalate exist in the slurry inthe hydrate decomposer, they will promote the generation of new crystalnuclei of alumina hydrate at the time of its decomposition and lower itsagglomeration activity, reducing, as a result, the grain size of aluminahydrate.

As a method to remove the above impurities, according to the US. Pat.No. 3,372,985, all the alumina hydrate seed is filtered and washed intwo stages. In the second stage of washing, sodium oxalate is dissolvedin the washing liquor and removed, and then caustic soda is recovered bytreating the washing liquor. But, as this method filters and washesgreat quantities of solids, it requires equipment of large scale and avast volume of washing liquor.

A method is also known in which impurities are separated and removed byevaporating and condensing the circulating spent liquor (clear overflowfrom the hydrate thickener in the final stage) and caustic soda andalumina are recovered by roasting the separated impurities. This method,however, requires not only evaporating and roasting equipment of a largescale, but also a large amount of heat.

The main object of this invention is to provide a method which, in theBayer process, easily removes impurities, especially sodium oxalate fromthe spent liquor and recovers caustic soda or sodium oxalate by treatingthe removed impurities.

As a result of various investigations and researches, the inventors havefound a method to remove, in the Bayer process, impurities comprisingmainly sodium oxalate and recover caustic soda or sodium oxalate bytreating the removed impurities by means of utilizing the fact that i.the solubility of sodium oxalate in a caustic soda solution containingsodium aluminate varies in a wide range depending on the concentrationof caustic soda and the temperature of the solution,

ii. sodium oxalate which is crystallized from a liquid supersaturatedwith sodium oxalate is, when existing together with a large quantity ofalumina hydrate crystals, contained mainly in the finer part of thecrystals,

iii. sodium oxalate dissolved in a solution of a suitable alkalineconcentration reacts with slaked lime to form calcium oxalate.

1n the following detailed explanation of this invention, in theaccompanying drawings:

FIG. 1 is a graph showing the solubility of sodium oxalate in a sodiumaluminate solution of high alkaline concentration.

FIG. 2 is a graph showing the solubility of sodium oxalate in a sodiumaluminate solution of low alkaline concentration.

FIGS. 3 and 4 are schematic views showing the methods of this invention.

Referring to FIG. 1, the concentration of sodium oxalate is shown on theordinate axis and temperature of solution on the abscissa axis.

Ordinarlily, in the Bayer process, the alumina contained in bauxite isextracted by a circulating liquor containing caustic soda at theconcentration of 160 g./l. and then said circulating liquid is cooleddown to about C. and separated from solution residues to produce a clearliquor. Further, the liquor is cooled to the temperatures of 60 to 70C.to separate alumina hydrate and the liquor after removal of aluminahydrate is used again as the circulating liquor. Meanwhile, the liquorlowers in temperature to about 50C. by natural cooling.

Now, if sodium oxalate exists in the circulating liquor at theconcentration of l g./l., as shown in FIG. I, saturation is attained at70C. and, with further lowering of temperature, the state develops intosupersaturation and sodium oxalate begins to crystallize from theliquor. As the crystallized sodium oxalate is contained mainly in thefiner part of the crystals, the sodium oxalate may easily be separated.

In FIG. 2 are shown the relationship of a low concentration caustic sodasolution and its temperature to the solubility of sodium oxalate inwhich the concentration of sodium oxalate is plotted on the ordinateaxis and the temperature of the solution plotted on the abscissa axis.

It can be seen from FIG. 2 that if solids containing sodium oxalate aredissolved in hot water, sodium oxalate will dissolve as theconcentration of caustic soda decreases, but if caustic soda is added tosaid solution, the concentration of caustic soda will increase andsodium oxalate can be easily precipitated.

Referring to FIGS. 3 and 4, slurry from hydrate decomposer 51 is sent toprimary hydrate thickener 52. In thickener 52, an underflow with aconcentration of 500 to 700 grams of solid per liter is separated fromthe slurry. The underflow is then filtered by filter 53 and calcined bykiln 54 to obtain the product alumina.

Filtrate from filter 53 is mostly sent to tertiary hydrate thickener 57,but part of it is transferred through line 2 to classifier 59.

Overflow from primary thickener S2 is carried to secondary hydratethickener 55 and overflow from the latter thickener 55 is sent totertiary thickener 57 except for part of the overflow which istransferred to classifier 59 through line 1.

In classifier 59, the up-current velocity is adjusted so that thesolution can be separated into an overflow with a concentration of 2 to5 grams of solid per liter and an underflow with a concentration of 400to 700 grams of solid per liter. The underflow from classifier 59 iscollected in hydrate seed storage tank 56 through lines 8 and 4 togetherwith underflows from secondary thickener 55 and tertiary thickener 57and used as a hydrate seed liquor.

As stated previously, when the concentration of sodium oxalate is in astate of superstaturation, the crystals of sodium oxalate become veryfine. Therefore, the solids in the overflow from classifier 59 containsodium oxalate in percentages ranging from 10 to 30 weight percent.

This process is shown in FIG. 4 as A-process".

As is clear from FIG. 1, if the overflow from classifier 59 is cooled,it will become a solution supersaturated with sodium oxalate. If hotwater of 50 to 70C. is used as washing water in filter 53, almost allthe sodium oxalate attaching to the alumina hydrate will be dissolved.Further, if the filtrate from filter 53 is added to this washing water,the concentration of caustic soda will be increased by the filtrate andthe solution will be supersaturated with sodium oxalate. Besides, if apart of the overflow from tertiary thickener 57 is separated and cooled,similarly it will become a solution supersaturated with sodium oxalate.

Accordingly, if these solutions are cooled individually or collectivelyin cooler 60 to temperatures of 20 to 50C., transferred to precipitator61, given an underflow containing crystals of sodium oxalate as seedsthrough line 9 from classifier 62 and agitated for 2 to 3 hours, thesodium oxalate in the solutions will continue to precipitate until thesolution attains saturation.

If these solutions containing crystalized sodium oxalate are sent toclassifier 62 and said classifier is operated adjusting the up-currentvelocity to 2 to 5 m./h., the solids in the overflow from saidclassifier 62 contain sodium oxalate at a concentration of to 35 weightpercent.

This method is capable of obtaining crystals which contain more sodiumoxalate while treating a smaller quantity of solution compared with thecase where the A-process is applied individually.

This process is shown in F IG. 4 as B-process".

Next, filter 63 is used to separate solids from the overflow containingsolids which include large quantities of sodium oxalate from classifiers59 and 62. For this filter, any type of filter such as a pressurefilter, vacuum filter or centrifugal separator may be used.

The solids (including 10 to 40 percent of moisture) separated by filter63 are sent through line 12 to dissolver 64 which is filled with hotwater supplied from line 13, and dispersed and agitated therein. Sodiumoxalate and other water-soluble impurities will dissolve. Next, thesolution is transferred to thickener 65 and wherein alumina hydrate willsettle.

The quantity of hot water that is supplied by line 13 in order tocompletely dissolve the sodium oxalate in dissolver 64 differs with thequantity of caustic soda attached to the solids separated by filter 63and with the temperature of the hot water. This relationship is shown inFIG. 2. Any amount of hot water satisfying the conditions of this figurefor total solution of the sodium oxalate will be sufficient.

For the hot water used here, it is possible to utilize the recoveredsteam condensed water generated in the dissolving process of bauxite.

The solids collected in thickener 65 were originally floating in theoverflow from classifiers 59 and 62. However, as watersoluble impuritiescomposed mainly of sodium oxalate have been removed, the concentrationof the dispersion medium liquid is low and both the viscosity andspecific gravity are smaller than the mother liquor of the Bayer processand therefore the settling property is considerably increased andsettling and separation can be performed in a thickener of a small size.Alumina hydrate is made into an underflow with a concentration of 800 to1000 grams of solid per liter, sent to hydrate seed storage tank 56through lines 15 and 4 and used as the hydrate seed liquor.

As the alumina hydrate in line 15 has been classified by three settlingand separating operations, it is composed of very fine particles 80percent of which are below microns and can be used for various specialpurposes besides manufacturing aluminum by the electrolytic process.

This process is shown in F IG. 4 as C-process.

When the solids (including 10 to 40 percent of moisture) filtered andseparated by filter 63 are sent to dissolver 66 which is filled withcaustic soda solution supplied from line 16 and dispersed and agitatedtherein, the alumina hydrate in the solids will dissolve but the sodiumoxalate will remain as solids without dissolving owing to the highconcentration of caustic soda in the solution.

As the caustic soda solution fed from line 16 has a high concentration,it must be adjusted to a suitable concentration by supplying hot waterfrom line 16.

The solids floating in the solution of dissolver 66 are impuritiescomposed mainly of sodium oxalate and are filtered and separated byfilter 67.

The filtrate filtered by filter 67 is carried to spent liquor storagetank 58 through lines 19 and 5 used to supplement caustic soda to theBayer process so that the alumina hydrate dissolved in the solution canbe recovered.

As the filter cake obtained through line 18 is mainly composed of sodiumoxalate, it may be used as material for manufacturing oxalic acid orroasted to recover caustic soda.

This process is shown in FIG. 4 as D-process".

ln the overflow from thickener 65 of the C-process" are containedcaustic soda at a concentration of 15 to 20 g./l., sodium oxalate at aconcentration of to g.ll., alumina at a concentration of 7 g.ll. andsodium carbonate at a concentration of i0 g./l.

When this overflow is transferred to causticizer 68 through line 14 andagitated therein for reaction by adding slaked lime from line 20, mostof the sodium oxalate will react and turn into calcium oxalate andcaustic soda. At this time, part of the sodium carbonate in the solutionwill also react to change into solids of calcium carbonate and causticsoda. Besides, part of the alumina in the solution will react andprecipitate as calcium aluminate, but the quantity is not large.

In this case, in the place of slaked lime, any basic substance whichforms insoluble salts by reacting with sodium oxalate such as bariumhydroxide or the like can be used.

The reaction temperature, reaction time and the amount of slaked lime tobe added to causticizer 68 must be determined by selecting the mostadvantageous conditions from the fact that the reactions to be formed atthis time are the equilibrium reactions of sodium oxalate, sodiumcarbonate and sodium aluminate with calcium hydroxide and the individualreactions are dissociation equilibrium reactions as shown by thefollowing chemical formulas.

in this invention, treatments were give under the following conditions.

Temperature: 50 to 60C.

Addition of calcium hydrate: 200 to 250 weight percent of the dissolvedsodium oxalate Reaction time: 1 to 2 hours When treatment is conductedunder these conditions, 60 to weight percent of the sodium oxalate inthe solution is subject to reaction and turns into calcium oxalate.

The solution which has undergone the above treatment is a slurry mainlycomposed of caustic soda and containing solids of calcium oxalate andunreacted slaked lime. This solution is sent to thickener 69 to beseparated into liquor and solids. The slurry flowing out of line 21 isan underflow of a concentration of 600 to i000 grams of solid per literand may be discarded or used as raw material for manufacturing oxalicacid.

The solution 6 which has been freed of solids contains caustic soda at aconcentration of 20 to 40 g./l. Therefore, it is mainly used as washingwater for washing red mud in the Bayer process.

This process is shown in FIG. 4 as E-process".

A part of the overflow from thickener 65 of the C-process" is sent toprecipitator 70 and caustic soda is added to it from line 23. if theconcentration of the caustic soda in the solution of the precipitator isincreased, the solubility of sodium oxalate will decrease and sodiumoxalate will precipitate from the solution. Thus, the solutioncontaining crystals of sodium oxalate is transmitted to filter 71 andseparated into solids and solution.

The solids obtained from line 24 can be used as raw material formanufacturing oxalic acid or roasted to recover caustic soda. Thesolution freed of solids is sent to spent liquor storage tank 58 throughlines 25 and 5 and is used to supplement caustic soda to the Bayerprocess.

This process is shown in FIG. 4 as F-process.

According to this invention, if all or some of the above A through Fprocesses are combined with the Bayer process, it is possible to removeimpurities which are mainly composed of sodium oxalate which is aharmful component in the process, and in addition easily recover usefulcomponents in the impurities.

As the sodium oxalate contained in the solution circulating in theprocess decreases thus preventing the accumulation of sodium oxalate inthe seeds, the reactions at the time of the precipitation of aluminahydrate will be accelerated, scale sticking to the equipment willdecrease and the grains of precipitated alumina hydrate will be larger.

The following Examples are further illustrative of this invention, andit will be understood that the invention is not limited thereto.

When the operation was conducted combining the A- process" filter 63"C-process E-process illustrated in FIG. 4, the results shown below wereobtained.

The classifier 59 in Table l was adjusted so that the up-cur- TABLE1.APPLICATION EXAMPLE OF A-PROCESS Sodium oxalate in solids Flow SymbolFlow rate of Con- Flow in rate, solids, tent, wt. rate, Description Fig.4 mfi/h. kg./h. percent kg./h.

Liquid fed to classifier 59 1+2 120 4, 800 2. 9 138 Overflow from aboveThe overflow in Table l was further filtered at pressures of 3 to kgJcm.and the filtered wet cake was dispersed in hot water of 90C. to dissolvesodium oxalate in it. The sodium oxalate was then settled and separated,the result being as shown in FIG. 2.

TABLE 2.-APPLICATION EXAMPLE OF "(J-PROCESS" Sodium oxalate contained inSymbfil solids or solution Description Fig. 4 Flow rate Content Flowrate Filtered cake-.." 12 530 kgJh. 18.0 wt. percent 67 kg./h.

(wet). (dry). Overflow from 14 1.9 mfi/h. g.[l 57 kg./h-

thickener 65.

Table 4 shows an application example in which the 8- process illustratedin-FlG. 4 was continuously operated at the temperature of C. and at theaverage reaction time of. 3 hours.

When the overflow of Table 4 was further given the treatment of filter63" C-process" E-process", it was possible to finally remove sodiumoxalate at the rate of 69.5 kg./hrt

TABLE 4.--APPLICATION EXAMPLE OF B-PROCESS" Further, the overflow of theA-process" shown in Table l was filtered, the wet cake produced was fedwith hot water and caustic soda according to the D-process in FIG. 4 and5 agitated to dissolve alumina hydrate while keeping the mixture at thetemperature of 70C. The solution was then filtered at the pressure of 2to 5 kg./cm. with the results such as shown in Table 5.

TABLE 5.'APPLICATION EXAMPLE OF D-PROCESS" Sodium oxalate contained inConcensolids or solution tration Concen- Symbol of caustic tratlonDescription in Fig 4 Flow rate Content Flow rate soda of alumina Wetfiltered cake oi filter w---" 12 530 kg./h. 18(Yivt. )percent 67 kg./h

ry Hot water 17 3.1 mfilh Pure caustic soda solution.-- 16 1.4 mi /l1650 g./l Wet filtered cake of filter 67 i8 100 kg./h 90 wt. percent 63kgJh... 4 kg./h 3 kgJh.

r Filtrate of above 1 4.6 mi /l1" 0.9 g./l 4 kg./h... 200 g.,'l 80 g.ll.

TABLE 6.-APPLICATION EXAMPLE OF F-PROCESS Sodium oxalate (solid orliquid) Concen- Flow tration Symbol Content, rate, of caustic Desonptionin Fig. 4 Flow rate g./l. kg./h. soda Overflow from thickener 65.... 141.9 m. /h 30 57 8.9 g./l Caustic soda solution to be added 23 0.8 rnJ/h650 g./l Wet filtered cake of filter 7i (excluding part to be returnedas seed)"- 24 78 kg/h 54 4 kgJh. Filtrate from above 25 2.7 m. /h 1 3200 g./l.

Further, calcium hydrate was added to the overflow in Table 2 and leftto react at C. for 2 hours. After this, settlement and separation wereperformed and such results as shown in Table 3 were obtained.

TABLE 3. AP1' LielmN EXAMPLE OF E-PROCESS Sodium oxalate I In thisfigure is containe d the substance which turned into calcium oxalateafter being calculated in terms of sodium oxalate.

hi the above example, it was possible to remove sodium oxalatecontinuously at the rate of 45.5 kg./hr.

According to the above result, sodium oxalate can be removed at the rateof 63 kg./hr.

Further, the clear overflow of the C-process shown in Table 2 was taken,given caustic soda and agitated at 50C. after adding seeds of sodiumoxalate to it to precipitate the sodium oxalate which remained dissolvedin the solution.

Then, the liquid was filtered at pressure of 2 to 5 kg./cm. with theresult such as shown in Table 6. According to this method, sodiumoxalate can be removed at the rate of 54 kg./hr.

Table 7 shows examples of alumina hydrate precipitated respectively bythe conventional method and the method of this invention.

TABLE 7.GRAIN SIZES OF PRECIPITATED ALUMINA HYDRATE Weight percentageBayer Convenprocess tlonal according Bayer to this Grain size processinvention Below 20 microns 10 3 20-40 microns... 30 15 Over 40 microns.60 82 What is claimed is: 1. in the Bayer process whereinalumina-containing raw material is digested in a caustic soda solutionthus forming a caustic soda solution pregnant with alumina hydrate whichalso contains sodium oxalate and from which is separated large particlesof alumina hydrate which are subjected to filtration to produce afiltrate and a filter cake of solid particles of aluminum hydrate whichare washed and calcined to produce product alumina and the liquorcontaining fine particles of alumina hydrate from which the largeparticles of alumina hydrate have been separated is further divided in afirst stage by sedimentation and separation into an underflow pregnantwith alumina hydrate and an overflow pregnant with sodium oxalate whichunderflow pregnant with alumina hydrate is recycled as a seed, and whichlast named overflow is combined with at least a portion of said filtrateto produce a liquid which when subjected to sedimentation and separationin a second stage yields an underflow which is combined with thepreviously obtained underflow and an overflow containing sodium oxalate,the improvement therein for removing sodium oxalate from said overflowscomprising:

subjecting at least a part of the overflow pregnant with sodium oxalatefrom said first stage wherein said overflow still contains a relativelylarge amount of alumina hydrate and another portion of the filtrateobtained by the above mentioned filtration to controlled up-currentvelocities in a classifier to produce an underflow with 400 to 700 gramsof solid per liter and an overflow with a concentration of 2 to 5 gramsof solid per liter, thereby increasing the amount of sodium oxalatecontained among the solids in the overflow from said classifier;

thereafter separating solid particles of sodium oxalate from said lastnamed overflow, cooling the overflow from said second stage to 20 50C.and thereafter separating sodium oxalate therefrom.

2. The process of claim 1 including, in addition, separating saidoverflow from said classifier into filtrate and filtered cake, treatingsaid filtered cake with caustic soda solution thus converting said cakeinto sodium aluminate solution and sodium oxalate solids and recoveringsaid sodium oxalate solids.

3. The process of claim 1 including in addition:

separating said overflow from said classifier into filtrate and filteredcake;

treating said filtered cake with hot water thus separating said filteredcake into alumina hydrate solids and sodium oxalate solution;

precipitating sodium oxalate solids in said sodium oxalate solution byadding caustic soda solution thereto; and recovering said precipitatedsodium oxalate.

4. The process of claim I including, in addition:

separating said overflow from said classifier into filtrate and filteredcake;

treating said filtered cake with hot water thus separating said cakeinto alumina hydrate solids and sodium oxalate solution;

causticizing said sodium oxalate solution by the addition of a basicsubstance selected from the group consisting of calcium hydroxide andbarium hydroxide thereby converting said sodium oxalate solution intocaustic soda solution and calcium or barium oxalate solids, the additionbeing about 200 to 250 weight percent of the dissolved sodium oxalate insaid solution; and

recovering said calcium or barium oxalate solids.

2. The process of claim 1 including, in addition, separating saidoverflow from said classifier into filtrate and filtered cake, treatingsaid filtered cake with caustic soda solution thus converting said cakeinto sodium aluminate solution and sodium oxalate solids and recoveringsaid sodium oxalate solids.
 3. The process of claim 1 including inaddition: separating said overflow from said classifier into filtrateand filtered cake; treating said filtered cake with hot water thusseparating said filtered cake into alumina hydrate solids and sodiumoxalate solution; precipitating sodium oxalate solids in said sodiumoxalate solution by adding caustic soda solution thereto; and recoveringsaid precipitated sodium oxalate.
 4. The process of claim 1 including,in addition: separating said overflow from said classifier into filtrateand filtered cake; treating said filtered cake with hot water thusseparating said cake into alumina hydrate solids and sodium oxalatesolution; causticizing said sodium oxalate solution by the addition of abasic substance selected from the group consisting of calcium hydroxideand barium hydroxide thereby converting said sodium oxalate solutioninto caustic soda solution and calcium or barium oxalate solids, theaddition being about 200 to 250 weight percent of the dissolved sodiumoxalate in said solution; and recovering said calcium or barium oxalatesolids. >